Creating a planet Atmosphere (Blurry Edge)
Final:



Overview:



In my setup I have used two objects - planet and rocket (created by t4k1t) and for lightning - Sun Lamp and Environment Map (HDRI).

Both objects are on separated layers.

In Render Layers tab you need to create two Render Layers and set it up accordingly to your object layers. Note that I've turned on Environment pass for Planet layer to have some background. You can skip this if you are using other method for it.

Turn on Transparent in Film section under Render tab.

After rendering your scene go to Composite and here are the blocks from screenshot explained:


Glare + Lens distortion for "blur" effect.
Combined and Inverted layers Alphas.
Combined layers.
Combined layers with Environment pass.


If you are not using HDRI map you can skip 2 and 4.